The present invention is directed to a contact arrangement for a vacuum circuit breaker which has a pair of contact members and an arrangement for generating a magnetic field in an axial direction including current loops with each contact member having at least one contact surface for making contact with the contact surface of the other of the pair of members, said current loops generating different axial magnetic field strengths in the radial direction of the contact arrangement.
A vacuum circuit breaker, which has a contact arrangement of a pair of contact members with one of the members movable on an axis of the arrangement, means for generating a magnetic field in an axial direction including current loops in each of the contact members, each contact member having at least one contact surface for making contact with the contact surface of the other of the pair of members and the current loops generating different field strengths in the axial direction, is disclosed in U.S. Pat. No. 4,196,327 whose disclosure is incorporated by reference thereto. As disclosed in this patent, behind each of the contact members are current loops which generate regions of higher and regions of lower magnetic field strength in an axial direction in the contact member and in the contact surfaces. The high magnetic field strengths in neighboring regions may have an opposite polarity.
These vacuum switches are distinguished by a rapid dielectrical re-solidification of the contact-break distance after the zero-axis crossing of the current or the breaking of the arc. This advantage of the vacuum switches can have a negative influence given unfavorable circuit data or, respectively, electrical conditions in the network, for example, when motors, which are starting up, are disconnected. Premature zero-axis crossing of the current having great steepness can, namely, occur and leads to the phenomena which are referred to in the literature as "multiple re-ignitions" and which can lead to disruptive overvoltages given breaking currents &lt;1 kA in the network.
In order to keep this effect low, the contact-break distance in the prior are is influenced such that the breaking current strength is kept low, whereby the overvoltages are reduced in accordance with the general opinion. This is achieved by means of an appropriate selected contact material of so-called "low-surge material". Examples of this material are composite chrome-copper materials with a bismuth or tellerium additive. Particularly because of their high vapor pressure, however, such materials only have a relatively low maximum breaking capacity or capability. Re-ignition also easily occurs here when breaking currents which, for example, amount to more than 10 kA occur and thus the shutoff is not for sure.
In British Patent Specification 1,598,397, a vacuum circuit breaker is disclosed with a contact arrangement where contacts with a rotating arc are divided into a main contact part and an auxiliary contact parts. The main contact part of the two contact members are brought into contact with one another when the switch or circuit breaker is closed but the auxiliary contact parts are not. Due to the current management in the contact disclosed therein, the arc is to be displaced from the main contact part into the auxiliary contact part. The main contact part comprises a low-surge material and the auxiliary contact part comprises a material for a high breaking capacity. As a result of the special shaping of these contacts, a weak current only generates an arc in the region of the main contacts herein the arc of a strong current will migrate to the auxiliary contact part where it encounters a material having the high breaking capacity. The electromotive forces that act on the arc occurs due to the current flux and the contact shape.
Due to the unstable behavior of the arc burning contracted given high current strengths, however, it is not always guaranteed that this arc will leave the main contact surface quickly enough that a thermal overload thereof is reliably excluded.